A murine xenotropic type C retrovirus was shown to contain in the viral core a low molecular weight DNA derived from host cell nonrepetitive sequences, perhaps adjacent to the provirus integration site. Adrenal cortical carcinoma was found in a high percentage of aged, ovariectomized NIH Swiss mice but not in age-matched, non-ovariectomized controls. Our previous studies indicated that expression of endogenous virus in certain target tissues of the mouse is under hormonal control, but there was no evidence of expression of viral proteins in the adrenal tumors by a variety of assays, nor was there measurable serum neutralizing activity against the endogenous virus. During gestation the NIH Swiss uterus viral reverse transcriptase activity appeared primarily in the microsomal fraction and could be described by a U-shaped curve with maxima at gestation day one and one day postpartum, whereas, p30 was primarily mitochondrial and described by a similar curve when expressed as ng p30 per microgram uterine protein.